1. Field of the Invention
The present invention relates to an image forming apparatus and an image processing apparatus and, more particularly, to an image forming apparatus for forming an image by scanning a rotating photosensitive member with modulated light based on an image signal, and an image processing apparatus for supplying an image signal to the image forming apparatus.
2. Related Background Art
A laser printer for forming an image on a sheet-like recording medium by scanning a photosensitive drum with a laser beam is known.
FIG. 8 is a view showing the arrangement of an optical scanner generally used for such a laser printer.
A semiconductor laser 80 as a component of a scanning optical system using a laser beam in the optical scanner is driven by a laser drive signal from a laser drive circuit 81 in accordance with an image modulating signal to emit a light-modulated laser beam. The laser beam emitted from the semiconductor laser 80 is incident through a collimator lens 82 and a cylindrical lens 83 on a rotating polyhedral mirror (to be referred to as a polygon mirror hereinafter) 85 driven by a scanner motor 84. The incident light is deflected by the polygon mirror 85. The laser beam deflected by the polygon mirror 85 serving as a deflector is formed into an image by an f-.theta. lens 88 constituted by a spherical lens 86 and a toric lens 87. Thereafter, the direction of the optical path of the light is changed by a reflecting mirror 89, and the light is irradiated on a photosensitive drum 90 which rotates at a constant speed.
The photosensitive drum 90 is sequentially scanned with this laser beam in a main scan direction a upon rotation of the polygon mirror 85 and in a sub-scan direction b upon rotation of the photosensitive drum 90 at predetermined timings. With this operation, an electrostatic latent image corresponding to the light-modulated laser beam is formed on the photosensitive drum 90 whose surface has been uniformly charged. This image is developed into a toner image by a developing device (not shown). The toner image is then transferred onto a sheet-like recording medium fed in the sub-scan direction b, and subjected to a fixing process, thereby completing the image formation process.
The laser beam from the polygon mirror 85 is reflected by a horizontal synchronization mirror 91 placed at a predetermined position outside an image formation area and detected by a horizontal synchronization signal monitor photodiode 92. As a result, a BD (Beam Detect) synchronization signal (to be referred to as a horizontal synchronization signal BD hereinafter) is generated. The timing of a main scan operation in a horizontal direction parallel to the axial direction of the photosensitive drum 90 is based on the horizontal synchronization signal BD. This horizontal synchronization signal BD serves as a reference signal for the timing of each process in the laser printer.
In addition, the horizontal synchronization signal BD is input to an image processing unit 93 for performing drive control on the scanner motor, image signal control, and the like to be synchronized with an image processing clock for image signal modulation which is generated by a crystal oscillator, thereby performing timing control for the start of image formation. That is, an image signal input to the image processing unit 93 is output, to the laser drive circuit 81, as an image modulating signal containing information associated with an image write start timing in the main scan direction which is controlled in accordance with the image processing clock. A laser deflection/scan operation is then executed by the route described above.
If timing variations occur for each main scan operation in the process of image formation start timing control, a print dot pattern is distorted. As a result, high-quality printing cannot be maintained. For this reason, image formation start timing control is performed on the basis of the horizontal synchronization signal BD, as described above. With this control, in sequentially performing a main scan operation, a first printed dot pattern is free from distortion.
A monitor amplifier 94 detects a current signal corresponding to the amount of light received by a photodiode (not shown) arranged near the emission area in the semiconductor laser 80, and performs APC control (Automatic Power Control; automatic light amount control) on a laser drive signal from the laser drive circuit 81 on the basis of this current signal such that the amount of light emitted from the semiconductor laser 80 becomes a standard light amount.
In the above case, a reference oscillator such as a crystal oscillator for oscillating/outputting an image processing clock is required to establish synchronization with an image processing clock for image signal modulation by using the horizontal synchronization signal BD and perform image formation start timing control. The horizontal synchronization signal BD is detected at an asynchronous timing with respect to the output phase of the reference oscillator. For this reason, an image modulating signal must be generated at the timing of an image processing clock for image signal modulation which is generated on the basis of the horizontal synchronization signal BD and can control the image formation start timing.
In order to control the image formation start timing with a precision of 1/n dot, the phase error between the horizontal synchronization signal BD and the image processing clock must be set to 1/n or less of the image processing clock period. The following conventional method has therefore been proposed. In this method, a highly stable reference oscillator such as a crystal oscillator is used, and the oscillation frequency is set to n times (n is a positive integer) the image processing clock frequency. An oscillation frequency Q times the clock frequency is frequency-divided in synchronism with the phase of the horizontal synchronization signal BD to generate an image processing clock for image signal modulation. With this method, the phase error between the horizontal synchronization signal BD and the image processing clock can be suppressed to 1/n or less of the image processing clock period.
In another case, a clock selection means is used. This clock selection means uses a delay line element having a plurality of output taps for a plurality of output signals with different delay times. In this arrangement, an image processing clock is input to the delay line element, and the phase of the clock is sequentially delayed in units of 1/n the image processing clock period. The resultant clocks are output to the respective output taps. Of these outputs, the tap output exhibiting the least phase error with respect to the phase of the horizontal synchronization signal BD is selected. The phase error can be reduced by using the selected clock as an image processing clock having undergone phase correction.
In the former case, if, for example, the image processing clock frequency is set to 18 MHz, and the phase error allowable value is set to 1/16 (n=16) dot, a crystal oscillator serving as a reference oscillator is required to have a high oscillation frequency of 448 MHz. It is, however, difficult to obtain a crystal oscillator having such a high frequency. Even if such an oscillator can be obtained, it costs too much.
In the latter case, since the delay time of the delay line element greatly varies, the delay time of each tap of the delay line element must be managed to realize a phase error of 1/n dot or less. In practice, however, it is difficult to manage the delay time. Even if delay line elements which can realize an allowable phase error can be obtained, the yield of such elements is poor.
Assume that image data is to be subjected to pixel division modulation at intervals of 1/k (k is a n arbitrary integer) of the image processing clock period. In this case, when parallel image data input at the image processing clock period is sequentially converted into serial data and output by using a clock having undergone a 1/k phase shift, which is selectively output from the clock selection means using the delay line element, jitter corresponding to the variations of the delay line element may occur in the image data having undergone pixel division modulation. For this reason, only delay line elements which are free from variations and hence from phase errors may be selected and used. In this case, however, since the yield of these components is poor, the apparatuses become very expensive.
In both of the foregoing cases, a pha se error may be caused between the horizontal synchronization signal BD and an image processing clock owing to the finishing precision of the polygon mirror, and the image formation start timing of each scan line may be shifted, resulting in jitter in the formed image in the main scan direction. For this reason, only polygon mirrors with high finishing precision which do not cause any phase error may be selected and used. In this case, however, the apparatuses become very expensive.